VM Snapshots

VM Snapshots

VM snapshots capture the complete state of a virtual machine at a point in time: disk contents, memory state, and CPU registers. This makes them ideal for before-and-after testing: take a snapshot, make a change, test it, then either commit or revert. Snapshots in KVM/libvirt are stored as QCOW2 backing chain entries — each snapshot records only what changes relative to the previous state, making them space-efficient.

Snapshot types in KVM

Creating snapshots

# Create a snapshot of a running VM (disk state only):
virsh snapshot-create-as ubuntu-test   --name "pre-upgrade-$(date +%Y%m%d)"   --description "Before apt upgrade"

# Create snapshot including memory state (freezes VM briefly):
virsh snapshot-create-as ubuntu-test   --name "pre-upgrade-with-mem"   --memspec snapshot=external   --atomic

# List all snapshots for a VM:
virsh snapshot-list ubuntu-test

virsh snapshot-list output

 Name                 Creation Time               State
------------------------------------------------------------
 pre-upgrade-20250609  2025-06-09 14:30:00 +0000  running
 post-install          2025-06-09 15:00:00 +0000  shutoff

# Show snapshot details:
virsh snapshot-info ubuntu-test pre-upgrade-20250609
virsh snapshot-dumpxml ubuntu-test pre-upgrade-20250609

Reverting and deleting

# Revert VM to a snapshot (rolls back all changes since snapshot):
virsh snapshot-revert ubuntu-test pre-upgrade-20250609

⚠️ WARNING: Reverting a snapshot is destructive — all changes made after the snapshot was taken are permanently lost. Verify you have the right snapshot name before reverting. If the VM is running, it will be stopped and reset to the snapshot state.

# Delete a snapshot (removes snapshot but keeps the VM):
virsh snapshot-delete ubuntu-test pre-upgrade-20250609

# Delete all snapshots for a VM:
for snap in $(virsh snapshot-list ubuntu-test --name); do
  virsh snapshot-delete ubuntu-test $snap
done

Conclusion

VM snapshots are most useful for risky operations on test environments: database migrations, major package upgrades, or configuration changes where you want an instant rollback. For production VMs, prefer backup-based recovery (disk image copy + database dump) over snapshots — a snapshot chain that grows large can significantly slow VM I/O as QCOW2 must traverse the backing chain for every read. Delete snapshots after you've confirmed the operation succeeded.